Google launched an extensions gallery for Google Chrome and added support for extensions in Chrome 4 Beta (Windows and Linux only). Google also released the first beta version of Chrome for Mac, but it doesn't have support for extensions.
The extensions gallery includes 371 extensions. You shouldn't expect to find complex extensions, toolbars, download managers: Chrome extensions are lightweight, they have a minimalistic interface and few of them are customizable. There's a Gmail extension that displays the number of unread messages from your inbox, an extension that lets you subscribe to feeds, a Google Reader notifier, a Google News button, an ad-blocker, a Google Tasks button, extensions for StumbleUpon, Picnik, eBay, WOT, Twitter and Facebook.