One major collectable category is Souvenir Sheets. they've been around for quite a while, but in recent years there have been more and more of them. as with many things in the world of stamp collecting, they're less about postage and more about the collection.
Here is souvenir sheet from the US in 1947, commemorating the first US postage stamps.
Note that the stamps embedded in the sheet are valid for postage, you can cut them out and use them, or put the entire sheet on a package. But they're imperforate, you have to cut them out to use the individually. Early stamps were commonly imperforate.
Sheets become more elaborate over time. The stamps can usually be separated out and used more easily (why would you want to?) and the surround usually has lots of extra stuff. In the early 90s the US did a five sheet WWII series. The following sheet is the one for 1943 (of course we ignored 1939 and 1940, we're the US after all):
Both the 1947 sheets and the WWII sheets are shown mounted in a Scott Album. In these cases, Scott provided space to mount the entire sheet.
Sometimes all a souvenir sheet does, though, is provide a fancy border or label on a normal sheet of stamps. Here is a fairly recent
issue of DC Super Hero stamps:
Again, Scott conveniently allows the entire sheet to be mounted. And this is where the mini-rant about erratic Scott editorial choices begins. Here is another fairly recent superhero issue, this time dedicated to The Batman:
There are 8 distinct stamps here; the grid of 4 different Batman stamps - and then the 4 round bat signal stamps. So what does Scott do?
Why, they want me to cut my pretty souvenir sheet up. Idiots.
But then, the USPS is not always helpful. They did a Civil War sesquicentennial series of 5 sheets. But embracing the self-adhesive stamp era, they of course double sided the sheets which is pretty neat, except there's not a good way to mount them in an album with both sides showing.
But that's ok, because Scott only provides spaces for the two individual stamps, rather than the entire sheets. Here is the front of 1862 and the back of 1864:
So the resolution seems to be to get some blank Scott pages to insert into the album at the correct location for these sheets. As for both sides, well, maybe i'll end up buying two copies of each sheet and mounting with both sides showing. Sigh.