One of the first and foremost mods that I really feel that I personally absolutely can't live without is some form of HUD. I actually tried running my Rogue on Zangarmarsh without a HUD at first (installed Taeo's UI, which I still think is truly gorgeous) but I found that I'm far too addicted to having my Combo Points up front and center now. I've stuck with Drathal's HUD for a while now, and despite flirting with many others like Arc and Phaytal's, I just haven't broken off from that one as yet. But no matter if your bars are pretty and pointed, blunt and bordered, or just straight lines, having your health, energy and combo points up front and center is good stuff for Rogues.
Another in the list is Omen. I'm sorry KTM, but... I've found another Mod. Our love affair just wasn't meant to last. Omen has really come to fit the bill for me. Sure, it feeds to/from KTM (which I can't express enough gratitude for, because otherwise I wouldn't have half the people I've run with in meters) but when you get into a party with everyone using Omen is when it really shines. It has really become the force-to-be-reckoned-with in terms of threat meters, in my opinion. Unless you're spending all of your time soloing, or you like spending lots of close-and-personal time with the floor, this mod should be in your UI. And speaking of floor time -- for raiding Rogues, Deadly Boss Mods can be a real life-saver. There's nothing quite as shameful as when you wind up eating floor during a boss encounter when it could totally have been avoided with this handy little mod. (And the "pizza timers" thing just cracks me up.)
One I like to have to monitor performance is some form of Damage Meters. For me, I keep SWStats nestled into my FuBar at the top of my screen, and find that it works well for what I'm looking for in an everyday run. I'm also fond of SWStats as I don't have to worry about people being out of range and not being tracked. As with Omen, the more people who use it in your group, the better.
My next "must have" is some form of timer for Sap/Cheap Shot/etc. To that end, I currently use CCwatch, but I'm not married to that choice and am still researching options. It's handy enough, and I don't mind the footprint, but I suspect there are probably some better choices out there so far as having a timer to ensure that you know just when your crowd control will break. The important thing is to have a timer in there -- as with all these other mods, the specific mod you choose is really best determined by what you want your UI to look like, and how much memory you want to expend on your options. (I'm currently sizing up ClassTimer, but just haven't made the jump yet.)
Speaking of what you want your UI to look like, I have to admit, I loaded up my game, found myself staring at the base WoW UI bars the other day and was horrified. I can't remember the last time I saw those gryphons, frankly. With any luck, I won't have to see them again anytime soon, either. I jumped back out, flipped on Bongos and away I went. I'm fond of Bongos primarily because I like lots of scalable buttons being available should I need them. Of course, I'm also all about the "less is more" UI scheme -- you'll notice that on my primary Rogue's UI, I stress keeping as much as I can off the screen at all times. Now this isn't to say that I don't have quite a few mods going (heck, look at all the buttons on the minimap!) but I keep most of them set to where they don't come up unless I'm either in-combat, or in a raid. The rest remain nestled in FuBar and the heck off my screen until I want them there.
So there you have it. A quick and dirty listing of the mods (or types of mods) I couldn't live without as a Rogue. Now this isn't to say it's an end-all-be-all of Rogue moddery by a long shot, but I do know that playing would be a pain in my arse without these few around. What mods do you Rogues feel are ones you absolutely couldn't live without? Leave a comment, and let's compare mod notes!