I'd have to say that Delosperma (ice plant) is one of my favorites; I have a couple of other sedums that I'm fond of, but have to keep corralled.

I don't know if you would consider these "aggressive" but they do multiply rapidly...I love cannas and bearded irises. Also, I love some of the annuals that like to reseed a lot, the ones that I've noticed recently in my yard are sweet alyssum, johnny jump ups and phlox drummondii. I even had some johnny jump ups that started growing in my front lawn, which is bermuda grass on heavily compacted clay soil - I hadn't been too good about deadheading some that I had in planters near my front door. I got the biggest kick out of them (at least until DH cut the grass for the first time a few weeks ago and promptly took care of the "weeds" for me) :)

I call those fast spreaders "ground covers."
:)
And my very favorites are :
ajuga
leriope
monkey grass
aspidistra
wild violets

I love violets, but they're TOO aggressive for me. There's no such thing as a few violets!

I'll take perennial candytuft. It reseeds and spreads, but doesn't take over.

One that comes to mind right off hand is Black Eyed Susan Vine. I had a hanging basket with it years ago, just one mind you, and I'm still pulling the vines out of my beds. That stuff seeded itself everywhere and is so hard to control, but it is a beautiful vine for late season bloom.
Jan

Sweet woodruff. Some people say it's very aggressive, but I think that must depend on the variety. I have found it to be a wonderful shade ground cover. lacy and delicate. It spreads well, but isn't big enough to overwhelm anything but the smallest plants. It's great around hostas, Salomon's seal, coral bells, etc.

Here is a link that might be useful: Sweet Woodruff

I have a love/hate relationship with Black and Blue salvia. Sends underground runners everywhere but I just chop off their little heads when they pop up. It's blooming now, just beautiful.
Um purple coneflower, to think that I used to take the seedheads in the fall and spread them around the flowerbeds. Easy to pull up or if the seedlings get too big they can be relocated. Beautiful with purple leaved cannas as a back drop.
I just planted a sweet autumn clematis, I may be adding to my post later. : )

Wisteria. We've had ours for many years but now the roots are beginning to run underground. It is getting scary.

I would also say Black& Blue Salvia (hummingbirds LOVE it too!), and also Butter & Eggs--the flowers are so pretty, but it will spread so easily. I put mine in a kind of natural area in back so it can do whatever it wants.

I bought an akebia vine several years ago after seeing it growing on some of the columns at the Summit. I have it on a fence where it has spread beyond my ability to control it. It also sends out roots that come up several feet away. I love the plant, but wish I could keep it where I want it!

The black and blue salvia is also one of my favorites. I found the forsythia salvia at a home my daughter bought and brought a piece home. It grows tall and blooms yellow in the fall, spreads every where, but it is gorgeous and pulls up easily.

I love some of the bullyish spreading roses. Great if you put them in just the right area. Also 'Silver Dragon' Liriope is a thug and you have to constantly pull up new plants as it will spread several feet underground and then pop up. But it has such a lovely color and can quickly fill in for a nice effect. Plus you always have some to share, just keep it in check. 'Limelight' Artemesia is another one like this. You'll pull it up by the handful each year when it spreads out of its designated area and it benefits you to whack off the tips now and again to keep it from going to seed. But it can grow 2 or 3 feet tall or be cut at a variety of heights to fit your filler needs, and adds that great splash of yellow and lime green to the garden. I don't mind thugs if they have relatively thick stems and are easy to grip and pull up. I don't care for some of the mints though because they are so thin and frail so the stems break easily when you try to pull them. I'm still trying to rid one bed in particular of some mint though I can see that aggressive quality might have served me nicely in other areas if I'd thought it through a little better.

Danielle

Danielle, I went on a Mint Massacre myself this spring. Spearmint and peppermint mixed together just taking over and smothering other plants. I'm still getting a few stray sprigs up, but have conquered it for the most part. So then I go and get about 8 different kinds of Monarda in a trade, LOL! More mint! Terry had given me some upright EEs several years ago, they are now getting used to having the sun on their faces again. They just couldn't overcome the mint.

"Mint Massacre", yes that's a great term lol. I have no idea what kind of mint I planted but just when I think I got it all it pops up inside a daylily or Iris clump. I didn't realize Monardas were a kind of mint. I used to have them at another house but honestly I couldn't stand the strong, pungent smell mine gave off. I've shyed away from monardas ever since because I still remember that smell from the red monarda I ordered from some mail order plant company back in the day. Maybe they don't all smell like that though.

Danielle