I manage over 100 blogs using this great software but pretty much use similar plugins on each.
There are many plugins that do the same thing and some times it is a process of trial and error, installing, activating and deactivating until you find one that is right for you.
So I thought I'd share my list to make it easier for everyone:
1. Wordpress Database Backup - Helps me sleep much easier at night. Daily backups in my email what more can I say.
2. Platinum SEO Pack - Don't know what I'd do without this great plugin. My blogs rank so much better with it running. This plugin is highly customisable and a must have.
3. Google XML Sitemaps - Like #1, this is very handy for getting better results in Google and does all the work for you
4. Share This - Great way to generate some word-of-mouth action
5. Feedburner Feedsmith - Feedburner helps keep an eye on who is reading your RSS feeds.
6. Ultimate Google Analytics - Avoids your Analytics codes getting blasted away by updates and your stats stopping running
7. Get Recent Comments - Indispensible little plugin helps other users to see which posts on your site people are talking about.
8. Akismet - helps control that annoying comment spam.
9. Geo Mashup Plugin - a fantastic little plugin that makes it very easy to generate Google Maps. A little fiddly at first but once you get your head around it and get your API key it is very powerful.
10. EasyTube or Smart Youtube - six one or half a dozen the other. Basically easily embed YouTube into posts - only thing I hate about Wordpress is it makes it difficult to do this and Google Maps without plugins ...
Hope this helps ! Interested to hear if I've left some out that others find indispensable.