I have been using Seachem Replenish for about a month now (ordered from tastyworms http://tastyworms.com/seachem-replenish.html)
From the website:
"Restore and Maintain GH of salts designed to replenish physiologically relevant minerals that are removed by reverse osmosis or deionizing filtration. Replenish restores General Hardness (GH) using a balanced blend of both soft (sodium potassium) and hard (calcium magnesium) salts. Restoration of mineral content is essential since a complete lack of minerals will result in osmotic stress in those species whose osmoregulation systems are adapted to a mineralized environment. Severe osmotic stress can result in osmotic shock which will lead to rapid death.
Calcium (min)..................................................13.00%
Calcium (max).................................................14.00%
Magnesium (min)............................................1.00%
Potassium (min)..............................................0.12%
Sodium (min)...................................................0.60%
Sodium (max)..................................................0.70%
Ingredients: calcium chloride magnesium chloride sodium chloride potassium chloride"
My gh out of the tap is 4 dGH (71.6 ppm). This is actually not terrible, but a little lower than I would like. Roberts (2010) suggests that 50-100ppm is ideal for freshwater tanks. As Seachem states above, gh is important for osmoregulation. Here is a great little article by Steve explaining osmoregulation if you're interested http://www.raingarden.us/osmoregulation.htm
The instructions for Replenish say to add 1 capful (5ml) per 10 gallons to increase gh by 2.8 dGH. I have found this to be pretty accurate. Adding 3 capfuls to my 40 gallon tank gets me to 6 dGH (107 ppm). I have found this to be stable between my water changes so far. (I should also note that I tested the Replenish in a gallon of tap water as well because I use a buffer in my tank and was unable to say which was ultimately effecting my gh without testing the tap by itself, the Replenish worked as expected in the absence of a buffer)
Overall, I like this product. I do wish it were a little more concentrated... I can see I will be going through this stuff pretty fast, but it does what it claims.
Just a note: crushed coral will raise gh as well, and is certainly cheaper than using replenish. I have tried coral in the past and did not like having it in my filters and did not like the lack of control I had over how it effected pH/gh/kh. Because my kh/ph are so low out of the tap I was having to use a lot of coral to get a desired effect. I personally prefer to use a buffer to raise my kh/pH and the replenish to raise gh. I think crushed coral is a great option for a lot of people, but if it is not for you, Replenish is another possibility for raising gh! 
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