Whilst searching the Opera Mobile forums, I came across this fairly comprehensive list of free software for S60 phones. Some of the links are broken and there’s nothing on the development side, but a useful list nonetheless.
Also, if you’re excited by the prospect of Flash Lite 2.0 announced back in November, you’ll also be excited to hear from this official forum post that this will be available to developers from Macromedia Labs in January.
Update: Lost my phone recently. Here’s my list of the first five essential S60 apps I re-installed: