A simple addon that reports your experience values as a percentage and as numerical values each time you gain experience.  For example, if I'm a level one lowbie and I kill a critter to gain 15xp I will get the following message on my screen:
Suxtobemage's experience is at 10% (15/150) 135 to go.
The message is also duplicated to the chat medium I have chosen (default:/say).

Tip: A feature that I kind of reserve for myself is the /xps test command - it doesn't appear in ingame help but basically just fires off the "shout" function without needing to gain xp.  There may also be other "hidden" commands... ;)

Current Version: 1.42 beta

Ingame help:
/xps show    - shows the GUI (not yet implemented)
/xps throttle- sets the throttle (default: 0)
/xps setchan - sets the channel to output to (default: say) second argument is the target for channel or whisper (you must use a channel number) - eg '/xps setchan channel 1'
/xps showinfo- shows information (current channel, vers. etc.)
/xps sias    - toggles whether to show info at startor not.
/xps setwarn - toggles whether or not to output to UIErrorsFrame
/xps setcol  - sets the colour for the UIErrorsFrame output
valid colours: blue, red, green, black, white, yellow
or you can specify red, green then blue on a 0 to 1 scale
/xps help    - shows this help
-- Valid Channels --
say, emote, yell, party, guild, officer, raid, raid_warning, battleground, none (turns chat output off)

Note: The ingame help is incomplete in that the following syntax is also legal:
/xps <chantype> [<target>]
Which will output directly to <chantype>, if <chantype> is whisper or channel, it needs <target> to be specified as where you want to send it.  Specifying <target> with other <chantypes> will just cause XPShouter to silently discard <target>.

DONE:
  v1.42
 - Upgraded .toc interface number to patch 3.0.2 support
 - Added functionality: typing /xps <chantype> [<target>] will now output to <chantype> an XPS message.
 - Added functionality: typing /xps reloadui will reload the user interface (personal convenience)

vers. 1.4:
 - Add options for Channel and Whisper output.
 - Find a way to preserve preferences between sessions.

vers. 1.35:
 - Add colour customisation for the UIErrorsFrame message.

vers. 1.3:
 - Add options for experience gain thresholding (to throttle the addon when you're grinding or participating in some activity where XP is gained quickly in small amounts).
 - Add deficit to the output (eg 10% (10/100) 90 to go). (added in vers. 1.2 which was never released to the public).

TODO:
 - Implement the GUI.
 - Get user feedback for other features that could be implemented.

NEED:
 - Committed GUI specialist to create the XML for a GUI options pane.  I will edit the lua to allow for GUI implementation in the coming weeks.
 - User feedback for any features I have missed in either the current working version or the TODO list.