Stowe has a lot of natural snow bump trails.
Off of the Spruce Peak side, the shortest bump run I can think of is West Smugglers, off of the Sunny Spruce chair. It's marked black diamond only because I don't think it's ever groomed (i.e., it's not that steep). The bumps are usually shaped pretty well on it.
Off of the Sensation chair (Spruce side summit chair), there is Smugglers. I'm not sure if it gets groomed out periodically or not? The bumps never get very big, but I've never seen groomer tracks in it. Again, it's a black diamond only because it will have moguls on it, but it's not at all steep. It's a whole lot longer than West Smuggler's though. I've heard it called the "best learn-to-ski-bumps trail, anywhere". The only "catch" with Smugglers is that at one point (80% of the way down?) the trail takes a hard right with a bailout straight ahead... Take the bailout, as it gets a whole lot steeper and nastier if you don't.
Also off of Sensation is Whirlaway which is a royal pain to get into (there are a few rocks that never seem to get covered), but once you're past that, it's a fun rolling trail. It's a decent step up from Smuggs or West Smuggs, mostly because of the entrance.
On the gondola side... Perry Merrill (take a right as you get off the gondola) goes around a giant turn off the top. Skier's left (the inside) of the turn is allowed to bump up. The longer you stay in the bumps, the steeper it gets, but you can just traverse out at any time. There's also the first pitch on Chin Clip... probably about a dozen bumps long and then there's a bailout back to Gondolier. Both of those are pretty short though and then it's a long, long ways down on groomers back to the lift.
Off of the Fourrunner quad (Mansfield side). There's Gulch which has snowmaking (think: no rocks in the troughs!) and is much wider than anything else mentioned here. I find the bumps on Gulch to be pretty irregular though... Bump shape / rhythm is, to me, more important than bump size / steepness.
A lot of stuff below Midway is allowed to bump up as well -- Lower National, Lower Starr, Lower Lookout, and Lower Liftline usually aren't groomed to perfection. Getting onto Midway to get to all those is something of a pain though... Easiest way is probably Gondola, Perry Merril, Cliff Trail, Nosedive and then cut hard right across the bottom of National.
The sides of bunches of other trails will sometimes have baby bumps forming -- just look around. Powder days or warm spring slush days will pretty much get every trail to bump up to some degree. The rest of Stowe's bump runs (Chin Clip, Centerline and the Front Four trails) are a major step up in difficulty.
Outside of Northern Vermont... The North Face side of Mt. Snow has a lot of bump runs of varying "mean-ness" to them. Okemo also has a handful of trails with man-made bumps on them. I haven't skied either of those in years though, so I don't know the "progression" the way I do at Stowe.