Goodwill and other thrift stores are a great source for wall wart power moduals and +5vdc regulated cell phone charges, usually $1-2. Also old VCRs are a great source for electronic parts and are cheap sometimes.
There's waaay better stuff than that there if you spend some time looking, and go often.
Besides robosapiens and roombas (and Powerwheels), I've found large R/C tanks, trucks, cars, etc. One time I picked up a Wowee Roboquad for five bucks. I am always finding old Polaroid cameras for the ultrasonics (equal to the Senscomp 6500). I once found (but didn't buy) an electric two-wheel mobility platform (like a Segway, just cheesier) that would've made a good robot platform. Sometimes you find mobility chairs going cheap, too.
Let's see - high-speed air pumps from breathing treatment systems can be repurposed for pneumatics. I recently found a "professional" long-distance 802.11 can-tenna yagi for building-to-building use (paid $7.00 for that!). I've found various computer bits and pieces (once got a nice bluetooth Mac keyboard, also a laser barcode scanner, and an embeddable PC system). You can also find tons of wire and other such bits, plus remote controls of all sorts. There's cell phones and pagers to be had (sometimes you can repurpose the displays, or the vibrating motor). Playstation controllers and similar joysticks/pads for all kinds of fun. Large VHS video cameras - lots of fun stuff there (comeon - you know your robot needs a zoom lens!)...
Really - there is a ton of stuff to be had for electronics and robotics hacking from Goodwill and other thrift stores. I always go with an eye to see "what can I buy with 'hackable' parts"...
;D