Requirements: A compatible Android Device (look below)
Overview: It is an app you install from the market or by side loading, that has a single button that will root your device.
Should be 100% safe - nothing on disk is changed besides the root binaries, so if something goes wrong, simply rebooting the device should fix up any and all issues.
V1.2.2 : Adverts added (can be permanently disabled with menu option). Hopeful fix for devices such as Moment.
Here is the list so far of known compatible and incompatible devices. I haven't actually tested these myself - I only have a Galaxy S device - but this list has been created by reports from users.
Samsung Galaxy S (All variants)
Galaxy 3 I5800
Cricket Huawei Ascend
Droid 1 (z4root 1.2.1)
Garmin Asus A50
Desire (requires nand unlock)
Desire HD (requires nand unlock)
Evo (requires nand unlock)
G2 (requires nand unlock)
Archos 70 (unknown)
LG Optimus One (unknown)
myTouch 3G (unknown)
Download z4root.1.3.0.apk, upload your files and earn money.