Sit ups, push ups, leg lifts, running in place, walking - can be done inside. Tons of strengthening and stretching can be done inside. If you have extra $$, equipment can be bought to use inside. I guess I am missing part of the question? Do you mean stuff for free inside, or just in general?
I live in WA state, so I have to have options to do exercise inside or else I'd lose half my exercise days due to weather.
I'm a sucker for workout videos. 99% of them are SO ridiculous, but i just laugh my way threw them and it's worked for me! best part you can lock yourself in your room so no one can see you sweat it out!
I was really into exercise dvd's for awhile, and burnt myself out. Now there's only a few that I throw in now and then. I bought a fold up exercise bike for just $120 or so on amazon dot com and I really like it. I can watch tv and cruise, and it doesn't take up much room. And I have three sets of dumbells - 8, 12 and 15 and I do dumbell workouts every other night for just 20 minutes or so. Of course you can work out at home, many do and lose a lot of weight - and tone up. The books with routines on dumbells say they are better then gym weights anyways. And although they say cardio machines are expensive the one I found is just great. There's also jumping rope, walking in place, a mini trampoline... use your imagination.
OK this is silly and something to be done spontaneously but the other day I put something in the microwave that had to cook for 5 min. My son is 5 and decided to play a game with me chasing him around the house. I told him that if he could keep away from me until the "bell" rang he was the winner. If I caught him before that I got to tickle him. I know it's silly and goofy and not really an exercise but at the end of those 5 min my heart rate was up and we had a nice time. It's kind of our new thing anytime I use the microwave.
Kathy K. has a book Celebrity Fitness or something like that - and in the book she talks about how limited her time is, so she does exercises while cooking, squats in the shower, and other exercises while just doing house stuff...