Is an undergrad travel grant relevant to a CV? Last year, I presented a poster at a major conference. 
My lab didn't have the money to send me, so I applied for and received an undergraduate travel grant from my university's office of undergraduate affairs.

Is this a relevant and/or important thing to list on my resume for future scholarships and grad apps?
Giving a poster talk at a major conference is certainly important in your CV. If the travel grant to go to the conference was based on academic excellence, then it should also be included in the CV. If it was a lottery, or based on non-academic merits it might be irrelevant. Nevertheless, even in the latter case, it doesn't harm to put it on your CV as it shows some initiative and dedication.