You can download 9 great alpha transitions from Apple. These are the same alpha transitions that were release with Final Cut Pro 7/Studio 3. This was one of my favorite transitions in FCP7 and now its in FCPX.
Download the Alpha Transition Content from apple at this link here.
NOTE: This has been updated to support clips up to 5 seconds.
To install expand the ZIP archive and add the folder to the Motion Templates folder in the Movies folder of your home directory. Add Effects to the Effects folder and Transitions to the Transition Folder.
Some Tech notes worth mentioning. While adding all 3 types of clips allows for the Alpha transition to perform just like it did in Final Cut Pro 7, adding a Wipe Matte Only does not create the desired wipe only effect. To get the same wipe only effect that was possible in Final Cut Pro 7, add your wipe matte to the Wipe Matte drop zone and the Clip Alpha Matte drop zone. This will produce a clean looking wipe. Again this is only if you have a Wipe Matte Only and not any of the other clips. To create your own wipes using this transition, the “Wipe Matte” must go from White (Full Opacity of the outgoing Clip) to Black (Complete Transparency of the outgoing clip).