Long History short: My mother married my father (actually step-father (Dutch Citizen)) when I was 8 years old and proceeded to raise me as his own child. Now I am 26 and I was wondering if I was eligible in any form or way for Dutch Citizenship.
According to Wikipedia, the citizenship is only transmitted by the (biological) parents and, in the case of non-Dutch mother and Dutch father, the father must acknowledge the child before birth.
If you were a minor, you would fall in the following category for the citizenship by option (which is in practice a form of simplified naturalisation):
a minor who is acknowledged by a Dutch national who has been cared for and brought up by this Dutch national for an uninterrupted period of at least three years.
However since you stated you are 26 years old you are not under this category anymore.
Even if your father has legally adopted you, you are still not entitled to Dutch citizenship. However, you are exempted from the residency requirement if applying for naturalisation in the Netherlands. (I am not sure about this last sentence though, since Wikipedia apparently states this is true for people adopted after majority, which is not your case.)
So in that case if you are interested in the Dutch nationality you will have to apply for naturalisation. Bear in mind that this procedure, if successful, will require the applicant to renounce any other citizenship.