The G8 Game Timer sounds like what you're looking for, and Greg Aleknevicus has has a very nice review of it in The Games Journal. It seems like it might be hard to come by these days, as its out of stock in many stores,
though it appears the manufacturer still sells them directly.
From the review:
To my mind the device only really works well when playing games in which the players take sequential, non-interactive turns of several minutes duration. While this might seem rather restrictive there are lots of games that do fall into this category. Also, it's precisely these games in which a timer is most useful. The aforementioned Tikal is one such example and Vinci is another with which it worked quite well. Ultimately, I can't really say whether I recommend this or not. Not because I don't think it's a worthy piece of game equipment, it is. Rather, because its usefulness will depend so much on the particular game and group that's using it.
As mentioned in the review, a timer is less useful in certain games. Games that involve lots of interaction with other players during your turn, such as trading in Settlers of Catan, mean that the length of your turn doesn't depend only on how fast you play, but also on how fast your opponents make decisions. I don't think that Settlers of Catan would work well with a timer.
You also need to consider what you do when a player runs out of time; do you simply eliminate them from the game? In a two player game, when you run out of time you lose, and the other player wins; in some multiplayer games, elimination can disproportionately benefit one player over the other ones.
Then there are games in which people's turns generally go fast enough that the timer is more of a distraction than a benefit. I would put Carcassonne in this category; it doesn't usually take long for someone to place a single tile. If people are taking too long, there's a simple house rule that I use: people draw their tile at the end of their previous turn, instead of the start of the next one. That way, they have everyone else's turn to think about where they want to place it, and the board only changes a bit by the time it gets back around to them.
That said, there are some games in which the timer would be useful. Games in which elimination doesn't disrupt the game, which have long turns in which each player plays independently of the others, and which are prone to analysis paralysis.