Waist

Belt of Vim (AV,8,18,28)

In general, the two NADs you care about are Will and Fort: things that hit those tend to come with nasty kickers. Things that hit Ref, as a rule, tend to just be damage. This boosts fort by a scaling amount, and is thus a very interesting waist slot item. The problem is that for avengers, Fort is usually a lost cause until you can stack booster feats in Epic. You're just too far down for a +1 to +3 bonus to actually matter. Obviously, Str-primary avengers like it more: that puts you back in the curve and is actually pretty relevant.

Cincture of Vivacity (AV,14)

Only if you have a optimized healic in your party. Turn a surge into a ton of temp HP before you go off soling someone. Or, you know, you could help focus fire instead, and not need this as much.

Diamond Cincture (AV2,MM3,10,20,30)

For many other classes, this is the default waist item, but IMHO not for avenger. Repeating what I said above in the Belt of Vim section, Fort is a key defense, but normal avengers will probably have to chalk it up as a lost cause. On the other hand, that free healing is quite tempting, especially for those that actually do the classic avenger isolationist trick. And hey, if the occasional "4" actually misses your Fort, that's gravy.

Ironskin Belt (PHB,5,15,25)

Decent resistance. Starts on your turn, so use for OAs, you crazy Martyr.

Belt of Giant Strength (PHB,15)

Nice boost to damage, but only one attack. Still, +10 spike isn't horrible.

Belt of Sonnlinor Righteousness (D385,6,16,26)

Rated red because using this on a Revenant will cause the DM to proclaim "Rocks fall, y'all die". Better in RAW-Strict environments like Lair Assault

Belt of Titan Strength (PHB,26)

+10 to all attacks in a round. That should be at least 3. That will help chew through the thousands of hit points monsters have at these levels. (But if you've got 12+ attacks in the round, you'll chew through them faster)

Cincture of the Dragon Spirit (AV,6)

If you're strength based, better. But if you're not, you still may have enough strength to qualify for Power Attack , which means it will be enough of a boost to Intimidate that you might actually succeed. Which is good, because crazy guys with huge swords ought to be intimidating.

Girdle of the Umber Hulk (AV,15,25)

Burrow is like phasing. But everywhere you want to burrow? Solid rock. So the level 15 one won't be that useful.

Reinforcing Belt (AV,9)

Who's your DM? Will they actually do coup de grace s? If so, it's not red, but most won't.

Sash of Regeneration (AV2,28)

At the level you get this, it just cuts in half the healing surges you use at the end of the battle.

Sash of Vitality Ceaseless (AV2,14)

Nice boost to your effective healing surge value.

Wraith's Cord (AV2,30)

More items that would be cool, but you'll never use. Crits weaken is a great property, and the daily power is also handy ... although short lasting.