You can clear out your inbox by archiving older messages. That doesn't delete them; it merely moves them out of the inbox and into Gmail's archive (which is accessible by clicking All Mail in the left-side menu). Here's how:

Sign in to your Gmail account. Next to the Search the Web button up top, click Create a filter. In the "Has the words" field, type before:2011/01/01. That's just an example date; it would archive all messages received prior to January 1 of this year. You can use any date you want, as long as it conforms to the format YYYY/MM/DD. Click Next step. Check the box marked "Skip the inbox (Archive it)," then check the box for "Also apply filter to ### conversations below." Finally, click Create Filter. Depending on how many messages meet the filter criteria, the archiving process may take a minute or two.

When it's done, your inbox should be significantly smaller. Remember, you haven't lost anything; you've simply relocated old e-mail. Those messages will still appear when you search your Gmail account.

The other day I was reminded why is the greatest Web service, like, ever.