You wanted variable brightness, but with no mention of how many levels I'm going to suggest a
MillerMods Arc-AAA.
My favorite light is the LF2XT and the
MillerMods I just got has the ability to go even lower, is much smaller and lighter, comes on very low and "retains memory" due to the fact that as a simple twisty it's just a single twist to go to low, then immediately to high with no lag time.
I'm not looking at a Solitaire right now, but I feel that this is probably the best option for you. The one I have has the 50mA low setting, which is also available in 25mA. I'm supposed to send one of my Arcs in for a mod and the 25mA low is what I'm planning on. The one I presently have has the 240mA high, according to the sale thread I just bought it from. My opinion so far is that either the 25/50mA low and 240mA high is a perfect tiny light that is as addictive to hold as it is to use!
These have an absolutely beautiful beam and tint, which offers a gentle veil of light on low and the high level is so perfect for daily usage! They also hang very nicely for neck carry.
Although the pics have ceased to exist, this is still a very nice
review of the MillerMods Arc-AAA 2-level mod.