Here are some of the best resources for your retro computing:
Atari
Atariage.com – THE Atari resource. Includes forums for most retro computers and game systems.
Atarimania.com – Database of software & documentation for the Atari computers & game systems.
Tandy Color Computer
Colorcomputerarchive.com – Database of software & documentation for the Tandy Color Computer 1-3 & the Dragon line of computers. (currently offline being updated)
CoCoVGA.com – This is an excellent VGA adapter for the Color Computer 1-2 created by our own Brendan Donahe.
TI-99/4a
ti99resources – Database of software & documentation for the TI-99/4a sponsored by HLO & the Central Texas Retro Computer Society. (TBA)
ftp.whtech.com/ – FTP Database of software & documentation for the TI-99/4a and other retro TI computers.
https://atariage.com/forums/forum/164-ti-994a-computers/ – Atariage is where TI enthusiast go to hang out.
Arcadeshopper.com – TI-99/4a as well as other retro computer hardware resources for sale.
Apple II
Apple2online.com – Database of software & documentation for the Apple II line of computers.
Commodore Plus 4
Plus4world – Database of software & documentation for the Commodore Plu4 & C16.
MAME
mamedev.org – Pretty much emulates ever retro computer/game system out there.