Caution: Only use this tip if your computer is SECURE.
Recently, Judi Sohn wrote about a URL hack to save your Salesforce username and password (in an exposed, plaintext manner). This is a great way to save a lot of time logging into Salesforce.
There is another way to save un/pw combinations: Use Eclipse to store the un/pw/securitytoken. There's a catch (there always is): Once they're entered, you can't see them in plaintext for, say, doing a copy/paste of your token into Connect for Outlook, Excel Connector, etc.
For each client, I create a project in the Eclipse Force.com IDE, entering the username, password, and security token into the appropriate form. (This does require me to have a System Administrator, or at least "View All Data" and possibly "Edit All Data" profile permissions.) Then, instead of getting into Salesforce through the browser, I open a file in the project, such as an Apex Class, right-click, and select "Show in Salesforce Web." The browser does the rest, opening the org, and taking me to that Apex Class. It does not take me to the Home screen, but I'm willing to use that one extra click. Plus, it's nice to get directly into the Setup area sometimes.
That's it! Keep all your projects in Eclipse, and (as long as you're a developer and work in Eclipse a lot) it will save time.
To reiterate the above caution: Anyone who can open Eclipse can get into any of those orgs, so be careful with this! Always secure your computer. On Vista, require a password when waking from the screen saver, and use other security features whenever possible.