Alkylation of alkenes by nucleophilic substitution
The pi electrons in an alkene or alkyne can act as a nucleophile.  This class of reactions is often taught a bit later than the standard $\ce{S_{N}1}$ and $\ce{S_{N}2}$ reactions and comes up when neighboring group participation or anchimeric assistance is covered.

The pi electrons in a double or triple bond are not strongly nucleophilic but when you arrange things in their favor, pi electrons can act as a nucleophile.  The reactions involving double bonds that I am familiar with involve an $\ce{S_{N}1}$ mechanism.  Here are a few examples.



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In example (1) notice how the compound with the double bond solvolyzes $10^{11}$ times faster than the saturated model compound!  Note too that only the product with acetate group anti to the double bond is formed.

Triple bonds and benzene rings can participate as well.  Often in cases involving double or triple bonds new carbon-carbon bonds are formed between the unsaturated linkage and the carbon with the leaving group [examples (2) and (3) above]. In cases involving aromatic rings aromaticity is restored so no new carbon-carbon bonds are formed between the aromatic ring and the carbon with the leaving group.