If you've already set up a Google Account on your tablet:

1. Try entering unlock patterns several times right in a row. It *should* ask you to enter your Google Account info as an alternative way of letting you in, after which you can turn off pattern locking or set a new pattern.

2. If you set up a Google Account and the tablet isn't asking you for that info you can try to unlock it from this page:
https://www.google.com/accounts/DisplayUnlockCaptcha (this might only work on hardware authorized by Google, I'm not sure)

3. BUTTONS! There is usually some combination of button presses on boot that can get you into a "reset" or "safe mode" where you can reset it. It varies from tablet to tablet, however. For example, on my Haipad M701-R when I flip the on/off switch off and then on, short press the "end/sleep/power" button and immediately press and hold the "Menu" button through the boot process I go into "Safe Mode."

Also, some APAD tablets have a small, unmarked hole on the side of them that you can stick a paper clip into and press a soft-reset button. No idea if that clears an unlock pattern though.

4. If those options don't work, or if you have not set up a Google Account (either because you bypassed it or you have a tablet without official Google apps on it) another option is to use ADB (Android Debug Bridge) to unlock it. Go to Google and download the Android SDK (if you haven't already) and install it. If you're on Windows you'll also need the USB drivers.

1. Plug the tablet into the computer via USB and turn it on/boot it up.
2. Open a Terminal/Console and change directories to your Android SDK /tools/ folder.
3. Enter ADB commands. On Windows these will be "adb.exe" and on Mac/Linux "./adb" - I'll just use "adb" below for simplicity:

Code:
> adb devices
This will let you know if ADB can "see" and "talk to" the tablet. If you don't get any devices reported back check the USB cable, drivers (on Windows), and that the tablet didn't shut down on you. I should warn you, however, if the tablet didn't have "USB Debugging" turned on, this will not work. Assuming it reported back finding a device proceed:

Code:
> adb -d shell
# sqlite3 data/data/com.android.providers.settings/databases/settings.db
sqlite> update system set value=0 where name='lock_pattern_autolock';
sqlite> .exit
# exit
> adb reboot
If all went well the tablet should now boot WITHOUT the pattern lock enabled!

It will probably ask you for account info again like the first time it was booted. You can bypass that by pressing in all four corners in sequence upper left, upper right, lower right, lower left (if I recall correctly). Once into the tablet perform an actual factory data reset or it will pester you for setup info EVERY time you boot it. Go to Privacy>Factory Data Reset>Reset

5. Last resort is to reflash the tablet. Without knowing SPECIFICALLY which tablet model you have, though, there's no way anyone could tell you how to flash it or where to get compatible firmware. Assuming you DO know what you have and where to get a compatible rom for it, check elsewhere on the forums for instructions on how to flash your specific tablet with that firmware. Once it is flashed the "drive" is wiped and reset so you'll have to go through the "first run" setup again, but at least it won't have a pattern lock on it.