Once upon a time, the majority of loyalty-rewards points were earned for traveling. No more.

Today, more points are earned for credit-card purchases than for flying or staying in hotels or, well, for anything. Which explains why today's frequent-flyer programs are more accurately referred to as frequent-buyer programs.

When it comes to racking up credit-card points, the single largest trove of points is likely to be the sign-up bonus offered by the card issuer to new cardholders who meet certain requirements, typically meeting a spending target within a set period.

In recent years, such sign-up bonuses have occasionally topped 100,000 miles, and anything less than 50,000 miles is generally considered a yawner by consumers who zealously monitor bonus levels in pursuit of the most generous payouts. The offers are in constant flux, rising and falling as market conditions change and card issuers' goals evolve.

With that in mind, following is our monthly roundup of the biggest sign-up bonuses currently on offer for the most popular travel-rewards charge and credit cards. Bear in mind that these are limited-time bonuses confirmed to be in effect when this list is published, and could be reduced or eliminated at any time.

Citi / AAdvantage Executive World Elite Mastercard

Welcome offer: 75,000 miles after spending $7,500 within the first 3 months of account opening.

Annual fee: $450.

Comment: The high-end AAdvantage card comes bundled with a long list of travel perks, including annual lounge membership.

*Disclosure: Citi is an Airfarewatchdog advertiser

United MileagePlus Explorer Visa Signature Card

Welcome offer: 40,000 miles after spending $2,000 within the first 3 months of account opening, and another 10,000 bonus miles after spending $25,000 during each calendar year.

Annual fee: $95.

Comment: Card comes bundled with travel perks, including free checked bag, priority boarding, and 2 airport lounge day passes on cardholder's anniversary.

IHG Rewards Club World MasterCard

Welcome offer: 80,000 points after spending $1,000 within the first 3 months of account opening.

Annual fee: $49, waived the first year.

Comment: In addition to the generous bonus, the card features a low annual fee, waived the first year, and a free night after cardholder's anniversary.

JetBlue Plus Card

Welcome offer: 30,000 points after spending $1,000 within the first 90 days of account opening.

Annual fee: $99.

Comment: Card perks include free first bag, 10% rebate on points redeemed for awards, 50% discount on inflight purchases, Mosaic elite status after spending $50,000 annually.

Southwest Rapid Rewards Premier Credit Card

Welcome offer: 60,000 bonus points after spending $2,000 within 3 months of account opening.

Annual fee: $99.

Comment: Southwest periodically reprises this offer, the highest-ever welcome offer for this card. Points earned with the card count toward the airline's high-value Companion Pass. Cardholders receive 6,000 bonus points on their account anniversary.

Marriott Rewards Premier Credit Card from Chase

Welcome offer: 80,000 points after spending $3,000 within the first 3 months of account opening, plus an additional 7,500 points for adding an authorized user who uses the card within 3 months.

Annual fee: $85.

Comment: Good program, solid card, big bonus.

The Platinum Card from American Express

Welcome offer: 60,000 Membership Rewards points after spending $5,000 within the first 3 months of account opening. Terms Apply. Annual fee: $550. Comment: While the card's annual fee is among the very highest, it also comes bundled with a robust set of travel perks, including lounge access, $200 statement credit for bag fees, and 5 points per $1 for flights booked direct with American Express.

Chase Sapphire Preferred Card

Welcome offer: 50,000 points after spending $4,000 within the first three months of account opening, plus an additional 5,000 points for adding an authorized user who uses the card within three months.

Annual fee: $95, waived the first year.

Comment: Ultimate Rewards points can be transferred into a number of airline and hotel programs, as well as redeemed for travel through the Ultimate Rewards portal. In the latter case, 50,000 points are worth $625.

Starwood Preferred Guest Credit Card from American Express

Welcome offer: 25,000 points after spending $3,000 within the first 3 months of account opening. Terms Apply.

Annual fee: $95, waived the first year.

Comment: While the bonus itself is comparatively modest, Starwood points are among the most valuable loyalty currencies, as they're transferable to 30-plus airline frequent flyer programs, mostly on a 1:1 basis, and there's a 5,000-point bonus when transferring 20,000 points to a participating airline program.

Many of the above-listed consumer cards are also available as business cards, sometimes featuring the same sign-up bonuses. That means it's possible to double up on the bonuses if you're eligible for a business as well as a consumer card.

Editor's Note: This content is not provided by any bank, credit card issuer, airline, or hotel chain. Any opinions, analyses, reviews or recommendations expressed here are those of the author's alone, and have not been reviewed, approved, or otherwise endorsed by the aforementioned entities.