Yes, it's absolutely fine. You can also post it as an answer. You just need to make sure of two things:
- Your tool can be used to do what the question is asking for.
- You clearly mention that you are the author of the tool.
The 2nd is really, really important if you don't want to be taken for a spammer. You can read some more details here: How not to be a spammer. For example, this would be a bad, spammy answer:
This FOSS tool is great and does exactly what you need!
While this one would be OK:
I have written this FOSS tool which is great and does exactly what you need!
Or, simply:
This FOSS tool is great and does exactly what you need!
Note: I am the author of this tool.
Basically, as long as you clearly disclose your affiliation, posting answers to questions which your tool can answer absolutely fine.