Is selenocysteine C3H6NO2Se or C3H7NO2Se?
Selenocysteine can be either of the following: $\ce{C3H8NO2Se+, C3H7NO2Se, C3H6NO2Se-}$ or $\ce{C3H5NO2Se^2-}$. This is because the neutral form of selenocysteine is both a base and a diprotic acid: the amino group can be protonated and both the carboxylic group and the selenol can be deprotonated.[1]



The above graph taken from the chemicalize.com property calculation site[2] shows the distribution of the different forms of selenocysteine in aqueous solution. The underlying data has been calculated by chemicalize.com and is not experimental data. For the cell, neutral pH is most important. We can see that the most common form by far at neutral pH is $\ce{C3H7NO2Se}$, but a small quantity is present as $\ce{C3H6NO2Se-}$. Once selenocysteine has been incorporated into a protein, only the selenol function remains potentially acidic in aqueous solution; however, it’s $\mathrm pK_\mathrm a$ value is higher than neutral pH by three logarithmic units. Thus, we can assume that it will not be deprotonated notably.

The actual $\mathrm pK_\mathrm a$ values have been measured by Huber and Criddle.[3] These show that chemicalize.com’s predictions are rather bad. They determine the $\mathrm pK_\mathrm a$ values to be $2.01,5.24$ and $9.96$ for the carboxylic acid, the selenol and the amino group, respectively. Using these values, we expect selenocysteine to be present in the monoanionic form $\ce{C3H6NO2Se-}$ at cellular pH. A distribution graph based on these values (provided by Canadianer) can be seen below.



PubChem’s suggestion of $\ce{C3H6NO2Se}$ is wrong. If you add up valencies, you will notice that it is only possible as a radical structure. It would be acceptable if a negative charge had been included (which I don’t see on the site).

On the other hand while the chemical formula on Wikipedia or ChemSpider is correct and gives a neutral compound with all spins paired, this formula does not correspond to the species predominantly present in strongly buffered aqueous solutions.



Notes:

[1]: This way of writing it assumes the uncharged neutral form. In aqueous solution, the molecule is expected to adopt a zwitterionic neutral form with the carboxylate being deprotonated and the amino group being protonated. From this form, the carboxylate is the base while the amino group is one of the two acidic functions.

[2]: Unfortunately, I am not aware of a way to link directly to the calculation. However, this calculation is free if you register, since the molecule contains less than twelve heavy atoms.

[3]: R.E.Huber, R.S.Criddle, Arch. Biochem. Biophys. 1967, 122, 164–173. DOI: 10.1016/0003-9861(67)90136-1.