I have modified the standard bootloader to work on the 2560 mega.
http://www.cbxdragbike.com/arduino/bootloaderdocs/index.html
One important note, it only supports 128K bytes because the stk500 v1 protocol only supports 128k bytes.
Also, there is a bug in the gcc 4.3.2 compiler that really limits you to much beyond about 80K program space. This if fixed in gcc 4.4.0 and later.
I am working on an stk500 v2 bootloader for the 2560.
Mark