We're very happy with the tools we're using, but always open to new ideas. Anything you have that you're particulary proud of or happy with? Our arsenal includes many of the usual suspects.
Our startup is a web-based SaaS product, and I'd be interested to hear what tools make your startup life easier or more fun.
We are building a SaaS startup too. Here are the tools we use -
37signals.com - basecamp for project management, highrise for maintaining customer lists
Outright.com for free simple web-based accounting
Google docs (spreadsheets and documents) for collaborative editing. Also use Google mail for custom company specific email id. And for chat.
git for version control
jira for issue tracking
mailchimp for email marketing
wordpress for blogging
Also a SAAS based startup (enterprise finance/billing space)
Web based SAAS
I posted before about a couple of general business tools, and since you mentioned development tools, we use Beanstalk for our SVN repository and FogBugz for issue tracking. Google Docs serves as a convenient replacement for a wiki for us.
This is probably not relevant to you unless you use Ruby and Rails, but we decided to outsource a lot of our infrastructure so we use Engine Yard as our application platform, and we also use outside services to monitor the exceptions (Hoptoad) and performance (New Relic) of our application. In my opinion, it's more economical to spend money on outside services like this (and put up with the occasional problems) than to develop/maintain them ourselves.
Of course we also use Google's email on our domain, and Skype (it's more convenient than a phone even though we only make local calls).
Don't webtools limit your flexibility?
As a startup with mobility in mind isn't it important to be untethered?
Can you really do without excel, project,word or oneNote?
My 2cents on tools. check out Google voice.