There are many top 10/100 lists around (after all at the start of each year Digg's front page is just full of them) and CNET's Webware 100 is a great list of "web2.0" tools presented in a number of categories.

The Center for Learning & Performance Technologies co-ordinates a Top 100 Tools for Learning list which has inspired me to collate the top 10 tools that I use (in no particular order) on a regular basis and quite often within a normal working day:

1. Firefox - Just simply the best browser in terms of all around flexibility (yes there are faster and more stable browsers but firefox is a good workhorse). I try to keep the add-ons for Firefox to a minimum that include: scribefire, delicious, firebug and google notebook
2. Google - Default search engine within the browser although I am warming to Windows Live as the speed has improved and some searches just seem more relevant (try searching for your twitter tweets and compare results between Google, Yahoo! and Live)
3. Netvibes - This is set as my default homepage on all computers that I use - mainly for RSS feeds but also useful for other widgets 
4. Twitter  - Along with various desktop and iPhone clients: Twhirl, Tweetdeck, Twitteriffic & Hahlo
5. SugarCRM - A great open source CRM application although considering migrating to SplendidCRM as core supported web apps now on the .net/SQL Server platform
6. Development "toolbox" - Visual Studio 2008; SQL Server Management Studio and Coda for the mac;
7. Google Notebook - Great for snipping notes from web pages and collating together when simply bookmarking URLs via delicious is not good enough
8. Google Docs - Great for sharing and collaborating on small documents and spreadsheets although still not quite "there" for proper office productivity where Microsoft Office and iWork quickly take over
9. MS Exchange - I've had personal and work related Exchange accounts for years now and still prefer this as a platform in terms of functionality and "reliability"; GMail is great for archive email but Exchange remains the choice for current work related email
10. DotNetNuke - For CMS related activities this .net web framework has proven very useful over the past few years from Internet website applications through to Intra & Extranet implementations.

I'm pretty much 50/50 PC/Mac and all of the tools with exception of Visual Studio and SQL Server Management Studio are used on both platforms together with the iPhone.