Half-Life, Opposing Force, and Blue Shift are all awesome. If you liked any one of them, you will like the other two.
Blue Shift gets a lot of hate, and while I can understand being really disappointed with it when it first came out because it was so short, these days you can pick it up for less than the cost of a Happy Meal, and in my opinion it has some of the best action in the entire franchise (even though it is so short that I have never once taken more than one sitting to complete it). Unless there's some sort of horrible game-breaking element that I've somehow overlooked, it's totally worth your time and money (and it won't require much of either).