Wi-fi Analyzer - Great for optimising wi-fi coverage, checking out which open access points may be about etc. - Free version is good enough (ads can be disabled for a period of time), if I used more than occasionally then I'd definitely pay.
Last.fm - Great to have my last.fm library wherever I go.
Robo Defense - Tower defence game. One of the first things I downloaded when I got my Legend, paid for the full version almost straight away and am still playing it every day nearly two months later.
Dimmer widget - One click drops/restores screen brightness down to lowest level, great for nocturnal browsing/gaming.
Barcode Scanner - Checks prices for goods online when you scan a 1D barcode, links to websites and the Android Market with QR Codes. I'm really getting used to using this when out and about for comparing prices, it works really well.
LED Scroller Lite - Turns your phone display into a scrolling marquee of any text, much like the old Windows screensaver. Great for winding people up across the pub, or flashing rude messages to other cars (only joking)