I flash my G1 often and so I find myself re-downloading my apps frequently. Initially, i will always install:
Astro (file manager)
SetCPU / Wireless Tethering (need root, adjusts CPU frequencies/tethering over wifi)
Facebook
Twidroid (on 1.6, there is an official twitter app for 2.x)
Opera Mini 5 (fast, even on 128kbit)
Google Maps (the most up to date at the time, currently 4.4.0)
Movies (watch trailers, find theaters and movie times)
ShopSavvy (scans barcodes, tells you local and online prices0
Yelp
Google Shopper and Goggles
PayPal
Kayak
MSN Talk
Slacker/Pandora/last.fm
Work Email (exchange mail app ripped from the MyTouch3G)
Google Voice
IMDB
The rest are paid apps (games)
Uno
speedx
Bust a move
Diner Dash 2
Hyperspace
Light Racer 3D
Monopoly Here and Now
Oregon Trail
Tetris
Tower Bloxx
The Game of Life
and wunder radio which is paid.