For posting quick drawings here in the forum?
Doc
express pcb is easy to pick up and go, though I hate their board layout tool
fritzing works ok, and you get those goofy breadboard pictures no one understands as well
or you can gear up and use KICAD or Eagle, or any other semi-serious package
Let the quarterly flamewars begin...
It isn't like Vi vs. Emacs -- now that's a flamewar.
Doc, what OS do you use?
Kicad works pretty well for Linux, though I'm certainly not at a level to really test its capabilities.
it works pretty well on windows, though it has crashed on me once
I should mention that KiCad uses boxes, not zigzags for resistors. Might be there's a library for it that has the proper zigzag symbology. 
(If that's important to you.)
Old school: Graph paper, pencil/pen, and a scanner.
I use ExpressSCH ever since CrossRoads recommended it. Simple to use for forum diagrams, or documenting what you have done for your own info.
Quite a library of parts, and if the one you want isn't there you can "make" it fairly quickly, either by taking an existing one, "ungrouping" it, and changing it around, or just make one from scratch.
I'm a growing fan of www.upverter.com. Nothing to install and easy to share.
I am neither energetic enough nor am I of enough spare time to do learn to it with eagle or any of the others... I used Protel/Altuim for Many years but it takes so long to boot up (Ver 10) that I rarely use it, Multisim is for me quick and all I need for symbols but it is impossible as far as I can see to export a drawing from it... unless I missed something. Frequently in trying to explain the 'finer' points a picture might well help my 'Thousand Words"... or so. I guess I will try to find Protel 3 or 4.. either should be more or less in the Public Domain by now. As much as I enjoy helping and the few or three times I have been able to "See the Lights come on..." somehow isn't worth learning a tool that I will only use here... besides I personally think Eagle Drawings are Ugly... or at least don't look like anything I ever did except possibly 40 or more years age with a standard Drawing Template... Gawd those things (Pencil stencils) made terrible drawings Vi, Emacs... ? No use NotePad++ GNU Tu... Free Tu
Works very well though. I do thank all and sundry for an interesting set of answers. I don't have enough room on my desk top for a pad of paper and I can never find a pencil in a hurry or not a hurry... besides there is always someone who wants zig and zags (although there is no other part quite like it on the same drawing... except a pot... and that too is a). No pleasing every one . I read any and all, even some of that WWII Russian stuff and THAT was weird stuff too. almost like some of the patent drawings on "Radio Fuses" that were patented or used By the American Military in WWII. Yes I was born about 10 months after VJ day... (VJ day, August 15th 1945) and that officially makes me an Old Coot...
Doc
James Yes, thank you I will go and look at that now... Perhaps there is yet relief for weary eyes yet... I know If I posted drawings I wouldn't need to write quite enough and as much as I know that lessons especially long ones aren't really read, understood or appreciated by all that read them, perhaps I could make it less painful on my poor arthritic fingers. Because of that condition "Arthur has come to stay" writing is at times difficult for me. It's a finny thing but as much as my explanations tend to be heavy duty... Yeah, I know. I find that if it is possible every day tasks here can get so complicated... I was reading about someone that was concerned about microseconds time savings on a DS18B20 sensor with no apparent concern as to what he was going to accomplish with the saving in time, or for that matter whether the device could either??? between the latency of the sensor, it's limited range and the latency of any fixture that it might be a part of but he was trying. My point is not to criticize but simply to question the necessary oversight any one must employ in order to complete a job... Back in the Day we used to say when that happened (pointless modifications) that it "Was Time to Shoot the Engineer" or the guy that wanted the shooting gallery and the discussions about IR Remote sensors... All valuable to be sure and all interesting but I fail to see where they apply most of the time to solving the original question.
Doc
James I just signed up for the freebie, it looks good, kinda like Multisim... Thank You Sir, Now perhaps I can Shorten some of my posts...
Doc
Personally I am a big gEDA fan.
the proper zigzag symbology.
... Now that is flameable
ExpressSCH anyday, youll be up and running in 5 mins, simple and easy to use and learn.
If I just want a quick diagram, I actually find that searching for clip art on google and using cut & paste in gimp is faster than figuring out yet another user interface for a tool I'll forget how to use the next time I want to whip out a diagram :-).
Pardon me, but how are the 74HC595 & 74HC165 called in ExpressPCB? I can't find them.
Here's a similar Eagle bare minimum board...
The thread title asks for a 'Schematic Tool'. The diagrams produced by Eagle are more closely related to 'wiring diagrams', they are not at all useful as schematic diagrams.
Example:
If you look at pin 19 of the IC in the diagram in reply #3 you can see that it is connected to pin 6 of the 'Upper_Digital' connector. There is no clue that it is connected to anything else. You must scour the entire drawing (and hope you didn't miss anything) to find any other connections.
If you look at pin 19 of the IC in the diagram in reply #1 you can easily see that it is connected not only to pin 8 of Port B but also to pin 3 of the ICSP connector.
Schematic diagrams are designed to make the signal flow of a device easier to comprehend, wiring diagrams are designed to make the construction of the device easier.
Don
Yes I was born about 10 months after VJ day... (VJ day, August 15th 1945) and that officially makes me an Old Coot...
Maybe. From my viewpoint that makes you a 'babe in the woods'.
Don