Abonner for at downloade
SmartSnap: Extreme Precision

Credit goes to Syanide.

Very useful for aligning objects for various reasons.

This is a client-side tool for building contraptions with extreme precision, aligning objects for either aesthetic or functional perfection. To accomplish this task a grid is projected onto the object you are pointing at and by pressing the use-button your weapon will lock onto that point and allow you to place objects with pin-point accuracy. For all entities be them square, round or uneven this tool will bring you perfect precision.

Original thread (outdated since 2008):
http://facepunch.com/showthread.php?t=322155&p=5361641&viewfull=1#post5361641