APNdroid - A way to switch off all your data APNs in one go... bit extreme in some cases, but definitely cuts out unwanted data.
Barcode Scanner - Perhaps not the best, but the simplest way to scan a QR code

This allows you to take a photo of a QR code, and your phone will automatically read the data in it.
ConnectBot - Awesome ssh/telnet client.
Digital Clock Widget - (Yeah, there's more than one

)... Because analog is bad for you...
ES File Explorer - Seconded

Allows you to copy to/from SAMBA/Windows Shares.
GPS Status - Good info display for GPS. Combine this with AndNav2 (Not available on Market AFAIK) for a good offline map solution.
Phonolyzr - Some nifty graphs giving you details about your call history.
Task Manager - Ability to kill processes, as well as check memory and CPU use.
Twidroid - Great Twitter client.
Wallpaper Set+Save - Makes selecting wallpapers a bit quicker.
Watts - Great battery status app.
Wifi Analyzer - Quick way to check what wifi networks are close by, and a visual way to check channel overlap.
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I loaded the Cyanogen rom, and am still in the process of reinstalling most of these. Ringtone/Notification/Alarm sound management is a bit better on this, as well as the camera controls. I think I used Ringdroid on 1.5 for sound, but haven't found a free camera app that works as well as Cyanogen's.
another thing I no longer user is "Useful Switchers". Quick widgets to switch on/off WiFi, GPS, Bluetooth and 3G. Cyanogen has a neat power management bar that works quite well. Not as many options, but single-click for all I need.
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EDIT:
The app to make custom sounds MUCH better:
Rings Extended. If you select a sound, you get a choice to use it for selection, and it allows you to use just about any sound as an alarm, ringtone or notification.
A must have for 1.5!
Sorry, Ringdroid is a sound editor/recorder. Quite handy I'm sure, but I'd rather do editing on my PC.