Sure, people know they shouldn’t use the same password for their online accounts. But the fear of forgetting a password when it’s needed leads many people right back to this bad habit.

This week, I tested PasswordBox, a tool that stores account passwords—using the same level of encryption employed by the U.S. government—and automatically retrieves them whenever you open a website requiring a password. It happens seamlessly, so you don’t have to do anything other than open the website you want, as usual. PasswordBox is free for up to 25 passwords, or $1 a month for unlimited passwords. And it works across devices, so you’ll have an easy time using it on your computers, smartphones and tablets.

PasswordBox even offers a Legacy feature that lets you designate a digital heir, making password-protected accounts accessible to that person when you die. The heir must send the company a death certificate, which PasswordBox’s CEO Dan Robichaud says has happened several times without any problems since the company started offering the feature in 2012. I chose my husband as my heir and he was notified via an email invitation to the service, which he had to accept to complete this authorization process.

In the next two weeks, the company is expected to launch PasswordBox Wallet, an upgrade for its app on Apple’s mobile devices that will let users store information for government identification cards and credit cards in their PasswordBox accounts. In the two weeks after this update, PasswordBox will offer a free-for-life option for new users, giving them unlimited password storage for life. An Android version of PasswordBox Wallet will be available later this year.

I store passwords in Google’s Chrome Web browser as I go, logging into websites and selecting an option to let Chrome save my passwords. But that method doesn’t work as consistently across all websites as PasswordBox, forcing me to turn to my not-so-secure, old-school solution: a paper Rolodex of handwritten passwords, which I don’t always have with me.

PasswordBox synced all of my saved passwords as I logged into my iPad, Macs, Windows PCs and an Android smartphone. Though it took an extra second to open some websites, it worked well. I set up my account by creating one Master Password, which I used to log in for the first time on my devices. After that, the PasswordBox tool worked like a charm.

One thing I didn’t like about PasswordBox is the way it took over my Google Chrome browser settings. I have my “new tab” browser home-page view set to show Chrome apps, including TweetDeck, NPR, Google Drive, Hipmunk and about 30 others. Once installed, PasswordBox takes over your home page and shows icons of websites that are most popular among its users. This can be turned off by clicking on a small gear icon in the bottom right of the PasswordBox new-tab page and turning off the Enable StartPage option.

If you opt to keep PasswordBox’s StartPage, you can still see your Google Chrome Web apps by selecting a small Apps option at the bottom left of the screen, though these line up in one long, horizontal string and can’t be reordered according to your preference.

Another downside to PasswordBox is that while it works on major banking websites, it didn’t work when I opened my nationally known banking website. A message said, “Unfortunately, this site is not supported yet. Our team is working hard to support it, and it will be coming soon.” A company representative said banking and financial sites are always changing their form fields and PasswordBox uses technology to identify these changes and correct them as soon as possible.

After installing the PasswordBox extension for my Chrome Web browser, I saw a tiny green box appear in the user name and password lines of each website where I logged in. These green boxes serve as reminders that PasswordBox is capturing your passwords and saving them for future use the next time you log into that website. A message at the top of the screen tells you this is being done after you’ve successfully logged into the website account.

If you and a close friend or family member share passwords, you can use PasswordBox to do this with secure encryption rather than sending the password in an email or writing it down. For example, my husband sometimes uses my Amazon account because I can get free shipping with my Amazon Prime account, but whenever he uses it, he calls me to make sure he has the right password.

I selected Share New in PasswordBox, chose my Amazon account from a list and entered my husband’s email address to share its password with him. To accept this, he had to open a PasswordBox account, too, though. At any time, I can see who I’m sharing with and who is sharing with me, and I can select an option to unshare my passwords, too. If I want the actual password letters and numbers to be seen by the other person, I can make it visible, or keep it hidden.

Entering passwords can be nerve-racking, even when you’re browsing the Web on a secure home network. PasswordBox relieves a lot of these security concerns and works with minimal effort.

Write to Katie at katie.boehret@wsj.com