Another way to vary up your pullups is to vary what you hang from.
Gymnastic Rings are another useful pullup tool. Instead of grasping a nice round, often padded bar, you have to grasp a curved ring. The instability of the rings - they hang from stout straps and are not fixed in place - adds an additional challenge. This brings in more demands for stabilization, and makes it harder to gut out a last rep or too by squirming up one arm at a time. Rings are available online, such as these Elite Rings. The rings can also be used for other exercises, like pushups, dips, rows, and gynmastics practice.
Towels are an excellent way to work your pullup strength and your grip. Use a strong towel for this. If you go cheap and disposable, you're risking a tear and sudden fall. With a single towel, loop it over the bar, hold one side in one hand and the other side in the other hand, and pull up. Your range-of-motion is more restricted - your head will be in the way, or you'll have to duck it side to side (alternating between reps). Another alternative is to use two towels. Loop each towel over the bar, and grab both ends of each towel with one hand. Now you can pull yourself up normally (chin over the bar or chest to bar) without restricting your range of motion...but still taxing your grip and forearms.
A third alternative is to use one towel and the bar. Hold onto the bar normally - any grip is okay - and with the other hand grasp the towel. This makes for an uneven hand level; the towel hand will be lower by at least a few inches. The lower the hand on the towel, the more stress is placed on the higher arm. One way to progressively make these more difficult is to use a longer tower and grip it a little lower each workout for the same reps. Like any other uneven exercise, it's best to alternate sides between sets in order to even up the amount of work done by each side.
Towel pullups are a great way to improve your gripping strength, and the improved grip strength transfers nicely to bar pullups. One great thing about towel pullups is that you can pullup off anything strong enough to support you, even if it's too thick to grip, so long as your towel is long enough. I've done these on i-beams, square overhead beams, playground swingsets, and thick tree branches. As a bonus, you can wipe the sweat off with the towel when you finish.
Ropes can be used instead of towels. A thick rope can be hung between two overhead points and simple used as a flexible "bar." This makes for good grip training, but it is rough on the hands. Another way is to hang either a single rope or double ropes from an overhead pullup bar, and do pullups from them. Look at towel pullups and just substitute a rope.
I-beams are another often-overlooked place to do pullups. You'll need strong fingers, because you can't wrap your hands around an i-beam. You'll need to get your four fingers onto the flat portion of the beam and pull. Both chinups (supinated grip) and pullups (pronated grip) are possible. Mixed grip pullups are tricky because of the wide gap between the forward and back hands. Neutral grip pullups can be done, but you'll need to alternatively duck like on single-towel towel pullups. Be careful selecting your i-beam - it's unlikely to swap or break, but it might be dirty, greasy, rusty, or jagged-edged. Check carefully!
Bar thickness can be varied. One way to do this is with Tyler Grips over a regular bar. Another is to find a thicker, fatter surface, or wrap a bar with a towel. Lynx grips can also be used, providing a slight thickening of the bar and superior grip at the same time.
That's just a start to pullup variations based on grip and pullup station. You can get really creative. If you love pullups, you'll start finding yourself looking at everything in terms of "can I hang from that?"
Be careful when you get creative and/or desperate with pullup places. Overhead pipes in your home may be carrying hot water (giving a burn risk and a flood risk). Monkey bars on a playground are usually safe, but you'll still want to check. Same with chains on a playground swing. Swing crosspipes are great places for pullups, if your hands are sufficiently large to avoid injury from grasping such a thick pipe. Tree limbs can break, but they can also make for a great change of pace from a "real pullup bar."