Similarly engaging toys:
Meccano (http://en.wikipedia.org/wiki/Meccano), a French brand of metal rods, plates, gears, etc. that you can assemble together to form functioning small scale models (usually of vehicles). E.g.: http://upload.wikimedia.org/wikipedia/commons/f/f4/Meccano_0...
K'nex (http://en.wikipedia.org/wiki/K%27Nex), my personal favorite as a child- rods and plastic connectors that you can use to build vehicles, small scale models, toy guns, etc. I really liked them a a kid because while LEGO are more about building static dioramas/models, K'nex is more about building dynamic/usable contraptions. For example they had really cool kits to make solar powered robots that would crawl around and were very easy to modify; unfortunately these days they seem to be more focused on branded content (e.g. Mario Kart, Angry Birds).
Logiblocs (http://www.logiblocs.com), plastic blocks from the UK with electronic components encased in plastic that are easy to plug together, allowing kids to assemble projects such as alarms or a basic voice recorder. E.g.: http://www.logiblocs.com/images/understanding_spytech.gif . Their website seems to be stuck in the 90s :)
Littlebits (http://littlebits.cc), in a way a modern reinterpretation of Logiblocs - electronic components that snap together using magnets, sold as kits.
And of course Lego, but no need to talk about these :) Although the LEGO Mindstorms series should get a special shoutout.
Those are all the ones I can think of, but I'm sure other HNers will have contributions. I wonder if there's room to do a construction toy these days, given LEGO's titanic market and mindshare. I was very excited about Goldie Blox recently, but I thought it fell kind of flat - their models aren't very extensible/modifiable in the way that LEGO or K'nex or Capsela are.
"Unlike other building toys such as Lego, Construx feature beam-like pieces of varying lengths that snapped on to cubical connector knots in order to build large shapes. These are relatively secure even though no nuts or bolts were used. Panels allowed assembly of flat surfaces. Hinges, motors, wheels, and other movable parts expand the number of different shapes that can be built, and make moving creations possible."
I had two motors and could build simple robots. Due to the long size of the pieces, one could build truly gigantic things. I build really big helicopters, AT-T Walkers and whatnot. I played with this much more than with Lego. Also, the pieces had a bit of a 80ies future style look, which fit well for all the Sci-Fi things I was building.
But maybe I'm not :-)
I have a daughter and I bought her two capselas back when I wrote the blog post two years ago and she wasn't that interested. I think she expected a more espectacular outcome given the effort :)
I didn't have legos, as I mentioned I'm from Argentina and it wasn't very popular here, we had a clone called "rastis".
This year I went for the first time to US and we bought legos, but not a set, the $ 15 glasses of pieces. I did another trip in March and I bought more and last month a friend bring us another one. My daughter play with this every day.
I think legos in general and capsela are very different, but Lego as a brand has many different sets.
I agree with another commenter than "technic" is similar but it is a set and you know all the time what you should assemble and that's the reason I don't like Lego sets/models but I do like the pieces a lot.
The great thing about meccano was because it was made out of steel that it could take some amazing abuse - the downside was the nuts and bolts were very easy to lose. I ended up with a whole series of beaten up plates that I could not bolt together :)
The UK scene is quite active : http://www.skegex.nmmg.org.uk/
Lego Technic: http://en.wikipedia.org/wiki/Lego_Technic
Technic is a line of Lego interconnecting plastic rods and parts. The purpose of this series is to create more advanced models with more complex movable arms, such as machines with wheels, in addition to the simpler brick-building properties of normal LEGO.
For some reason they changed Lego Technic to studless beams (removed studded bricks) years ago. Lego has begun to re-incorporate studded bricks back into the Technic line - that's good.
However, I did get a set of Robotix: https://en.wikipedia.org/wiki/Robotix_(toys)
Looking at it now, I knew nothing of the comic book/TV show connection. I saw them at some children's science museum (maybe in Chicago?) and spent the better part of a day building cars and simple robots.
Robotix is definitely inferior to Capsela, because the only gearings it provided were the things necessary to run the sample project toys you could build, such as the grabber arm (use the slower, stronger motor for the hand!).