I accidentally uninstalled Google Talk using Root Uninstaller and now I can't download anything from the Market.
Can anyone help me? I don't have the .apk file for Google Talk.
The Google Talk application is a key part of the Android Market authentication. If you are not signed in to Google Talk, you cannot use the android market. This is one of the reasons there needs to be more "Warnings" with apps like Root Uninstaller. They allow you to remove vital apps from the device, and don't tell you what damage it could do.
You have a couple options:
You will need to find the apk for your device, or that works for your device. There are some devices that use a different version of the application, Then use a tool like Root Explorer for example, to put the apk file back in /system/apps
Restore from a Nandroid / Titanium backup (if you did the backup before hand)
(re)Flash a ROM on your device, which could put the device in the "factory state" if you are not flashing the same ROM that you already have on the device.
I also seen this in the applications description and it may actually make a backup of any app that you freeze or uninstall. check and see if you can restore with the application.
If you don't have a backup of the app, I would think the most sure fire and straightforward way to get it back would be to "restore to factory state".
Prior to doing that, I'd recommend using Titanium Backup to save all your settings and apps. Just reinstall TiBU again after you reset your phone and restore everything from the backup. Short and sweet.
If you decide you absolutely need to remove system apps in the future, Cyangogen mod has a bare bones guide on what's safe to uninstall.