I don't think there's any scientific alternatives to natural selection. (in response to pimephalis above)
Natural selection is the idea that if the characteristics of an organism allow it to survive better than other organisms, then it is more likely to pass on those characteristics to future generations.
The other 'alternatives' listed above (sexual selection and genetic drift) are just additional intricacies that are part of natural selection. Sexual selection is a consequence of the evolution of sex, which allows the rapid recombination of characteristics through the combination of genes (that is, the development of a set of characteristics expressed on any one individual is no longer dependent purely on chance mutation and selection, but through sex, characteristics can be combined and shuffled for each generation. Genetic drift is just noise that is inherent in the system. If there's no selective pressure, there will still be change, as long as it doesn't adversely affect fitness.
Evolution is something that has already happened. Natural Selection is a theory that tries to explain how evolution happened. An alternative to evolution is to ignore the evidence that it has occured.
Evolution and natural selection are truths, with a small "t". The kind of useful truth that is true whether you believe it or not. It may not make people happy if they are seeking Truth with a capital "T", the kind of Truth that make people feel better when they are about to die.